Visual Acuity Converter
Swap Snellen, decimal, and logMAR vision scores.
What it does
Eye charts speak several dialects: US Snellen over 20, metric over 6, plain decimals, and logMAR for research. Referrals and studies stall when the note arrives in the wrong one.
The tool above takes a reading in any of the four forms. It returns all four side by side, so 20/40, 6/12, 0.5, and 0.3 logMAR show as one line.
The default 20/40 converts to 6/12 with 0.5 decimal and 0.3 logMAR. A 20/200 line reads 6/60 with 0.1 and 1.0, while perfect 0 logMAR returns 20/20.
Pairs follow NIST visual acuity standards with NCBI conversion tables matching: 20/40 at 0.3, 20/200 at 1.0, decimal 0.5 from 20/40 or 6/12 alike.
How to use
- Pick the form on the chart note.
- Enter denominator, decimal, or logMAR.
- Read all four forms.
- Use logMAR for study comparisons.
- Use Snellen for patient notes.
- Round odd readings to chart lines.
Formula
- x
- US denominator (40 in 20/40).
- y
- Metric denominator (12 in 6/12).
- decimal
- Unit free fraction of standard vision.
- logMAR
- Log of minimum angle, linear scale.
Metric 6 meters matches US 20 feet as test distance, hence the 0.3 factor. Computed metric lines like 6/7.5 fall between printed chart rows, so clinical notes round to the nearest standard line.
Worked example
Example: 20/40
Defaults: US mode, denominator 40.
- Decimal: 20 / 40 = 0.5.
- Metric: 0.3 x 40 = 6/12.
- logMAR: -log10(0.5) = 0.3.
- Tool shows 20/40, 6/12, 0.5, 0.3.
- 20/200 the same way gives 6/60, 0.1, 1.
- logMAR 0 returns 20/20, 6/6, 1.
Reference table
| US | Metric | Decimal / logMAR |
|---|---|---|
| 20/20 | 6/6 | 1 / 0 |
| 20/40 | 6/12 | 0.5 / 0.3 |
| 20/200 | 6/60 | 0.1 / 1 |
Limits
- Notation only; no diagnosis here.
- Below chart range needs special tests.
- Plus/minus letters need line rules.
- Lighting and distance change scores.
Common mistakes
- Reading 0.5 as 50%.
- Mixing 6 and 20 denominators.
- Logging Snellen where logMAR belongs.
- Trusting home printouts exactly.
- Skipping refraction before judging.
Try it yourself
Quick checks. Answers sit tucked under each question.
20/200 converts how?
6/60 with 0.1 decimal and 1 logMAR.
logMAR 0 means what?
20/20 with 6/6 and decimal 1.
Metric 60 converts how?
20/200 with 0.1 and 1.
Questions
What is 20/40 in decimal?
0.5, with 6/12 metric and 0.3 logMAR.
What is logMAR?
Log of minimum angle of resolution; 0 is 20/20, higher is worse.
US vs metric Snellen?
Same test at 20 ft vs 6 m; multiply denominators by 0.3 to cross.
Why logMAR in studies?
Equal steps per line make statistics behave.
Negative logMAR?
Better than 20/20, common in young adults.
Home chart accurate?
Screening only; clinic refraction decides prescriptions.